1. Field of the Invention
This invention relates to a method for regenerating a denitrification catalyst for removing nitrogen oxides from a combustion exhaust gas from a boiler or the like.
2. Description of the Related Art
A method, put to practical use, for removing nitrogen oxides (NOx) contained in a combustion exhaust gas from a boiler or the like is an ammonia catalytic reduction process which decomposes NOx into nitrogen and water, both harmless substances, by use of ammonia (NH3) as a reducing agent in the presence of a nitrogen oxide removal catalyst (herein after referred to as “denitrification catalyst”).
In the treatment of an exhaust gas from a boiler using fuel oil C with a high sulfur content or the like as a fuel, a high concentration of sulfur dioxide (SO2) is present in a treated gas. Thus, the oxidation of SO2 into sulfur trioxide (SO3), which occurs simultaneously with the NOx reduction and removal reaction, generates a high concentration of SO3. The high concentration SO3 easily binds to the unreacted portion of NH3, which has been used as the reducing agent, in a low temperature region, thereby forming acidic ammonium sulfate and other compounds. These by-products cause clogging or partial obstruction to the interior and piping of various devices such as a downstream heat exchanger, etc, thereby increasing a pressure loss. Thus, it is necessary to take measures, such as upgrading of a dust collector.
As a catalyst showing excellent denitrification performance and low property of oxidizing SO2 (herein after referred to as low SO2-oxidizing property), a denitrification catalyst is available which has a tungsten oxide or a vanadium-tungsten oxide carried on titania.
However, dust, which contains heavy metals, such as vanadium, nickel and iron, and an alkali salt such as Glauber's salt, is present, in addition to SO2, in a combustion exhaust gas of a heavy oil with a high sulfur content. If the above denitrification catalyst is exposed to this exhaust gas for a long period of time, dust components are deposited and accumulated on the honeycomb-shaped wall surface of the denitrification catalyst, thereby inducing a decline in the denitrification performance and the enhancement of the SO2 oxidizing property. Thus, there is need to perform treatment for restoring the denitrification performance and the low SO2-oxidizing property of the denitrification catalyst.
The decline in the denitrification performance is mainly attributable to the fact that alkali components, such as Na and K, which are contained in the dust in the exhaust gas, are accumulated in the denitrification catalyst. The enhancement of the SO2 oxidizing property is mainly ascribable to the accumulation, in the denitrification catalyst, of vanadium which is contained in the dust in the exhaust gas.
Hence, the inventors proposed methods which comprise washing a used vanadium-tungsten-titanium based denitrification catalyst with water or an aqueous solution of a dilute inorganic acid, then cleaning the washed catalyst with a 0.1 to 5 wt. % aqueous solution of oxalic acid, and then further washing the cleaned catalyst with water to remove oxalic acid remaining on the catalyst, thereby removing the vanadium compound, the cause of the enhancement of the SO2 oxidizing property, so that the catalyst is regenerated (see Japanese Patent Application Laid-Open No. 1995-222924 (herein after referred to as Patent Document 1), No. 1998-156192, and No. 1998-337483).
Ultraheavy oils, such as ORIMULSION (a trade name of Mitsubishi Corporation for an oil-in-water type emulsion produced by mixing Orinoco tar, which is an ultraheavy oil collected in Orinoco State, Venezuela, with water and a surfactant for easy handling at the ordinary temperature), asphalt, and vacuum residual oil (VOR), contain 2 to 3 times as much sulfur, and 5 to 7 times as much vanadium as in fuel oil C, as shown in Table 1. Moreover, the SO2 concentration in the combustion exhaust gas of such ultraheavy oils is very high. When such ultraheavy oils are used as fuels for a boiler, therefore, a compound containing an alkali component, such as magnesium, is added to lessen corrosion in the boiler.
Hence, an exhaust gas from a boiler using an ultraheavy oil, such as ORIMULSION, asphalt, or VOR (hereinafter referred to as “ORIMULSION-fired boiler”), as a fuel is by far higher in SO2 concentration and dust concentration than an exhaust gas from a boiler using fuel oil C as a fuel (hereinafter referred to as “fuel oil C-fired boiler”).
TABLE 1Measured itemORIMULSIONOrinoco oilFuel oil CSpecific gravity—0.9760.889(80° C./4° C.)Viscosity—152834.8(cSt) (80° C.)N (wt. %)0.480.630.22S (wt. %)2.783.790.97Moisture (wt. %)28-30<0.10.1Higher heating value(kJ/kg)ca. 29400ca. 4200043680[kcal/kg][ca. 7000] [ca. 10000][10400]Ash (wt. %)0.080.130.01Residual carbon11.8917.48.0(wt. %)V (ppm)280-350400-500≦50Na (ppm)40-70 60-100—(Source: The journal “The Thermal and Nuclear Power”, No. 465, June 1995)
Thus, the ORIMULSION-fired boiler is much higher than the fuel oil C-fired boiler in the rate of deposition of dust on the surface of the denitrification catalyst. That is, dust fusion occurs under the action of sulfuric acid and liquid acidic ammonium sulfate which are formed when the temperature of the exhaust gas drops at boiler shutdown or the like. As a result, the amount of increase, per unit time, of the deposition on an upstream end portion of the denitrification catalyst in its exhaust gas flow direction, and on its surface in contact with the gas is so large that gas passage holes tend to be clogged and obstructed early.
In the event of an increase in a pressure loss due to clogging with dust, a decline in denitrification performance and enhancement of SO2 oxidizing property due to deposition and accumulation of dust on the surface and interior of the denitrification catalyst, oxalic acid cleaning as proposed by the aforementioned Patent Document 1 is performed to regenerate the denitrification catalyst.
No problem arises in regenerating the denitrification catalyst of the fuel oil C-fired boiler by the oxalic acid cleaning proposed by the Patent Document 1. However, if it is attempted to regenerate the denitrification catalyst of the ORIMULSION-fired boiler by the oxalic acid cleaning proposed by the Patent Document 1, dust deposited and accumulated in the denitrification catalyst is minimally removable, and the denitrification performance and the low SO2 oxidizing property can be recovered only to a certain degree.